The present invention relates in general to a method of forming a thin film transistor (TFT). In particular, the present invention relates to a method of forming a thin film transistor with a structure protecting gate electrode.
Thin film transistors are active elements used in active matrix type plate panel display and are usually used to drive active matrix type liquid crystal displays, active matrix type organic light-emitting displays, and image sensors.
In a liquid crystal display (LCD), a liquid crystal layer is sandwiched between two transparent glass substrates, and the thin film transistors are disposed on one of the transparent glass substrates. In the display region of the LCD, an array of pixel areas is defined by horizontally extended gate lines and vertically extended data lines. Each pixel area has a thin film transistor and a pixel electrode.
FIG. 1 is a cross section of a conventional thin film transistor, bottom gate type thin film transistor formed by back channel etching (BCE). The gate electrode 103 is deposited on the insulating substrate 102 using a physical vapor deposition (PVD) process and patterned by photolithography and etching. The gate insulating layer 104, the active channel layer 105, n+ type semiconductor layer 106, and a metal layer 107 are sequentially deposited in a chamber using a plasma enhanced chemical vapor deposition (PECVD) process followed by photolithography and etching to define an active channel region. A passivation layer (not shown) is deposited by a plasma enhanced chemical vapor deposition process to cover the active channel region. A transparent electrode is deposited by physical vapor deposition to serve as the pixel electrode (not shown) and the thin film transistor and its circuits are thus formed.
As liquid crystal displays increase in size, gate lines become longer, inducing signal delay and pulse distortion issues due to the high resistance of the gate lines. Thus, low resistance material is required for manufacture of the gate lines. Copper, a suitable low resistance material, is however reactive and difficult to use. For example, if copper is used as the gate electrode of the thin film transistor, when the copper gate electrode is exposed in an atmosphere of plasma enhanced chemical vapor deposition while the gate insulating layer is formed, the copper gate electrode easily reacts with free radicals from the atmosphere. If silicon oxide (SiOx) is used as the gate insulating layer, the silicon oxide film may detach. If silicon nitride (SiNx) is used as the gate insulating layer, bubbling may occur. Therefore, a buffer layer covering the copper electrode is needed before deposition of the silicon oxide or silicon nitride gate insulating layer.
U.S. Pat. No. 6,165,917 issued to Batey et al., teaches a method of protecting copper, aluminum or other metal gate electrode. After forming the gate electrode, an ammonia-free silicon nitride layer is deposited by plasma enhanced chemical vapor deposition to protect the gate electrode. The processing gas comprises silane/nitrogen/helium/hydrogen of 1:135:100:100.
US early publication No. 2002/0042167 to Chae et al., teaches covering a metal oxide layer on the surface of the gate line and the gate electrode using a thermal treatment to protect the gate line and the gate electrode.